1. Field of the Invention
Exemplary aspects of the present invention generally relate to a temperature control unit and an image forming apparatus including the temperature control unit.
2. Description of the Background Art
Generally, image forming apparatuses, such as a printer, a facsimile, and a copying machine, form an image including characters and/or graphics on a recording medium such as paper, cloth, an OHP sheet, and so forth based on image information.
There are different types of image forming apparatuses using different recording methods. In particular, an image forming apparatus using an electrophotographic method is widely used in offices, because fine images can be formed on normal paper at a relatively high speed.
Such an electrophotographic image forming apparatus is generally equipped with a charging device, a writing device, a developing device, a transfer device, a cleaning device, a charge-neutralization device, and so forth, which are disposed around a drum-type or a belt-type photoconductive image bearing member.
The image bearing member is charged while rotating. Then, an electrostatic latent image is formed on the surface of the image bearing member based on image information. The electrostatic latent image is developed with toner so that a visible toner image is formed on the surface of the image baring member.
Subsequently, the toner image is transferred onto a recording medium directly or indirectly, depending on the type of intermediate transfer medium employed. In a case in which the toner image is transferred through a belt-type intermediate transfer medium, the toner image is transferred indirectly.
After the toner image is transferred onto the recording medium, the recording medium is guided to the fixing device, where the toner on the recording medium is fixed. The surface of the image bearing member is cleaned by the cleaning device after the image is transferred, and the charge on the surface of the image bearing member is removed by the charge-neutralization device in preparation for a subsequent imaging cycle.
In such an electrophotographic image forming apparatus, a thermal fixing method is widely employed. In the thermal fixing method, a heating device such as a halogen heater or a ceramic heater heats and fuses the toner on a recording medium. The fused toner is then pressed against the recording medium so that the toner is fixed onto the recording medium. The thermal fixing method is preferred because a relatively fast fixing speed can be achieved, and the quality of the fixed image is relatively good.
However, the image forming apparatus using a fixing device using such a thermal fixing method tends to consume a significant amount of electric power in order to heat the toner. Further, a rise time of the fixing device using the thermal fixing method for starting the fixing process tends to be long in such an image forming apparatus.
By contrast, due to increased awareness of environmental problems in recent years, a fixing device capable of low electric power consumption is desired.
In view of the above, Japanese Patent Unexamined Application Publication No. Sho59-119364 and Japanese Patent No. 3290513 suggest, for example, a non-heating fixing method, in which a toner is dissolved or caused to swell by the fixing liquid and then dried so that the toner is fixed. Such a non-heating fixing method allows the toner to be fused without heat, compared to the thermal-fixing method using heat to fuse the toner. Thus, the non-heating fixing method is an ideal fixing method in terms of low electric power consumption and energy saving. It is desirable that the fixing liquid be stably supplied regardless of an operating environment.
However, a drawback of the non-heating fixing method is that the viscosity of the fixing liquid fluctuates depending on the temperature of the operating environment. Thus, when using a fixing liquid, the viscosity of which decreases as the temperature of the fixing liquid rises, the viscosity of the fixing liquid of this kind drops when the temperature of the operating environment is relatively high. Consequently, an amount of the fixing liquid supplied is most likely to increase, thus unnecessarily consuming a large amount of the fixing liquid and causing curling and/or cockling of the recording medium.
By contrast, the viscosity of the fixing liquid increases when the temperature of the operating environment is relatively low. Consequently, an amount of the fixing liquid supplied is most likely to decrease, thereby causing possible fixing failure.
In an attempt to solve such problems, Japanese Patent Unexamined Application Publication Nos. 2004-109751 and 2006-195429 suggest a method for controlling the temperature of the fixing liquid.
For example, a heater is provided in a tank containing the fixing liquid so that the temperature of the fixing liquid can be regulated. Further, it is also suggested that a heater be provided in an eject head, through which the fixing liquid is ejected, so as to regulate the temperature of the fixing liquid before the fixing liquid is ejected.
However, only that portion of the fixing liquid that is near the heater is directly heated in the above described methods. Thus, when the temperature of the operating environment is relatively low such as in a cold region or in the morning, the temperature of that portion of the fixing liquid that is relatively far from the heater is lower than that of the fixing liquid near the heater, causing the temperature of the fixing liquid to vary. As a result, the viscosity of the fixing liquid also varies, causing the supply amount of the fixing liquid to fluctuate.
In an attempt to solve the problem described above, one possible solution may be to provide an agitation device to agitate the fixing liquid while heating the fixing liquid. However, such a configuration tends to be complicated, thereby complicating maintenance as well. Further, there is a problem with this configuration including the mixing device in that the size of the device is difficult to reduce, thereby complicating efforts to make the image forming apparatus as a whole as compact as is usually desired.
In view of the above, in order to heat the fixing liquid evenly, it is possible to use a relatively large heater so as to increase the electric power of the heater, thereby making it possible to heat the entire fixing liquid. However, As described above, such an approach is undesirable in view of reducing power consumption and energy saving, since increasing the size of the heater simply increases the amount of electric power consumed, thereby defeating the purpose of using a fixing liquid in the first place.
In another aspect, in recent years, as electrophotographic image forming apparatuses have become widely used, a large amount of printed waste paper material and toner waste is discarded, leading to environmental concerns over the safety of such waste and contamination of the environment.
Conventionally, widely-used resins used in conventional toner include, for example, polystyrene, styrene-acrylic, polyester, epoxy, styrene-butadiene, and so forth. Generally, the toner including the resins described above is incinerated or buried in landfill when discarded.
Incinerating the toner including such resins requires a large amount of energy, thus easily damaging a furnace and shortening the life of the furnace. Moreover, toxic gases such as carbon monoxide, sulfur compounds, chlorine and so forth are generated in the process of incineration, contaminating the atmosphere.
In addition, because the resins described above have relatively high chemical stability, when buried in landfill the resins remain semi-permanently in their original form without being degraded. Therefore, such resins may adversely affect the environment and consequently pose a hazard to human health.
Furthermore, when such resins are discarded in the environment, the resins remain largely unaltered in the ground for an extended period of time, so that it is highly possible for wildlife to consume the resins inadvertently, which may disrupt local ecosystems.
Moreover, in order to recycle resources, one key task is to recycle or reuse normal paper. However, de-inking of conventional styrene resin is difficult to perform by conventional alkaline hydrolysis.
In light of the above, a toner including a biodegradable resin for toner that is harmless to human health and by definition biodegradable over time is proposed in Japanese Patent Unexamined Application Publication Nos. Hei04-218063 and Hei08-262796. However, such a toner is most likely to be used in the thermal fixing method rather than in the non-heating fixing method. Thus, it is desirable to be able to use a toner including a biodegradable resin in the image forming apparatus using the non-heating method.
However, although toner including biodegradable resin is fused or caused to swell in the fixing liquid more easily than the conventional toner, the fluidity of the toner is difficult to decrease. Further, toner including the biodegradable resin tends to have a viscosity that easily causes offset. For this reason, when using toner including biodegradable resin in an image forming apparatus using the non-heating method, consistent supply of the fixing liquid is particularly important.